Conventionally, a parking brake is used for restricting movement of a parked vehicle. For example, a manual parking brake is used in which an operation force applied to an operation lever is transmitted to a brake mechanism of the parking brake when the operation lever pulls a brake cable caused by the operation force. An electrically driven parking brake is also used in which a rotational force of a motor is transmitted to the brake mechanism of the parking brake.
In the electrically driven parking brake (i.e. EPB), a brake force is generated when, for example, a rotational force of a motor pulls a brake cable and the tension of the brake cable is thereby transmitted to the brake mechanism (i.e. actuator). In a lock control of the EPB, the parking brake control device rotates the motor in a lock side direction (i.e. forward direction) to transmit the rotational force of the motor to the brake mechanism (i.e. actuator) and stops the rotation of the motor with the brake force remained. In a release control, the parking brake control device rotates the motor in a release side direction (i.e. reverse direction) to remove the brake force.
Thus, the lock/release control includes the lock control and the release control. In the lock control, the parking brake control device keeps the EPB locked at a desired force by stopping the rotation of the motor in the lock side direction when the tension applied to the brake cable reaches a target value. In the release control, the parking brake control device detects that the brake force is removed by detecting that the tension of the brake cable becomes approximately zero. See Japanese Patent Application Publication No. 2001-514597 (U.S. Pat. No. 6,249,737).
However, in a built-in brake in which the EPB and a service brake commonly use an actuator, the actuator is under influence of the status of operation of the service brake. In other words, the actuator is under influence of a brake hydraulic pressure. Therefore, there is an occasion in which the brake force is not totally removed even if a tension sensor detects that the tension applied to the brake cable becomes zero. If the release control is terminated in this occasion, the brake force is not removed and therefore “brake force dragging” occurs in which a residual brake force remains even if a drive is not performing parking brake operation.
In the above description, the EPB uses the brake cable to transmit the rotational force of the motor to the brake mechanism. However, the EPB is not restricted to ones which use a brake cable to transmit the rotational force of the motor to the brake mechanism. For example, the EPB may generate a hydraulic pressure by pressing a hydraulic piston with the rotational force of the motor and may press brake pads or brake shoes with the generated hydraulic pressure. The EPB of this type also has a problem similar to that of the EPB using the brake cable. The EPB of this type which does not use a brake cable generates a pressing force for moving a friction material such as a brake pad and a brake shoe by rotating the electrical motor installed to the EPB. Then the pressing force generates the brake force. In this type of EPB, the parking brake control device can detect the pressing force for moving the friction material and terminate the release control when the detected pressing force is decreased to a target value at which the brake force caused by the EPB is supposed to be removed. By executing this type of control, the problem occurs which is similar to one described for the EPB using the brake cable. Even in the case that the EPB uses a brake cable, it is possible to execute the lock/release control based on the pressing force for moving the friction material and not based on the tension applied to the brake cable. In this case, the problem also occurs which is similar to one described for the EPB using the brake cable.